Inkjet printing is relatively inexpensive and is used in many different applications. In inkjet printing, droplets of ink are ejected from a printhead in response to electrical signals generated by a microprocessor and are deposited on a print medium, such as paper or polymeric substrates, to form the desired image.
When the print medium has a white recording surface, it is possible to print easily legible characters, signs and images in various colors. When it is desired to print on a print medium that has a recording surface that is other than white, however, or on a recording medium that is transparent or semitransparent, it may be desirable to print with white ink. By way of example only, white ink may be used in conjunction with inkjet printing to indicate positions for part mounting on printed circuit boards.
White ink formulations typically include a particulate white pigment dispersed in a solvent. One common white pigment is titanium dioxide, TiO2, also known as titanium(IV) oxide or titania. When used as a pigment, it is sometimes called “titanium white,” “Pigment White 6,” or “CI 77891.” Titanium dioxide is widely used as a white pigment because of its brightness and very high refractive index. Other white pigments include zinc oxide, and salts or esters of titanic acid.
White inks based on these compounds tend to have less-than-satisfactory characteristics, however. In order to provide optimal optical effect, the white pigments are provided as particles having an average size of at least about 1 μm, and sometimes larger. However, because the compounds are relatively dense, particles this large tend to settle and agglomerate. This is particularly disadvantages in inkjet applications, where the ink is stationary for long periods and where the printhead requires smooth-flowing, homogeneous ink. If an ink does not function properly in the printhead, image quality is reduced.
Likewise, other pigments that are typically provided as particles may suffer from similar settling and agglomeration traits. Thus, pigments that are sufficiently stable for use in other applications may be insufficiently stable for use in inkjet applications.